In the image forming apparatus of the electrophotographic type, as a charging means for electrically charging a photosensitive member (electrophotographic photosensitive member), a corona charger (hereinafter, also referred simply to as a “charger”) has been widely used. In Japanese Laid-Open Patent Application JP-A 2005-84688 and Japanese Patent No. 5382409, in a constitution using the corona charger, in order to meet speed-up of image formation or the like, a technique using a plurality of corona chargers and a plurality of grid electrodes has been proposed.
However, even when a plurality of corona chargers are used, in the case where a charging process of the photosensitive member having large electrostatic capacity is performed or in the like case, “charging non-uniformity” such that a charge potential of the photosensitive member becomes non-uniformity occurs in some instances. As a result, image defects such as image density non-uniformity and “roughness” due to a fluctuation in image dot occur in some instances.
On the other hand, in JP-A 2005-84688, a decrease in potential non-uniformity by using grid electrodes different in aperture ratio between an upstream side and a downstream side with respect to a rotational direction of the photosensitive member has been proposed.
Further, in Japanese Patent No. 5382409, a method in which two discharging wires are provided and voltages applied to the two discharging wires, a grid electrode and a shield electrode, respectively, are independently controlled has been proposed. However, in the conventional methods, it turned out that in a constitution in which a charging process of the photosensitive member is carried out by forming a combined surface potential by superimposing charge potentials formed by a plurality of chargers, it is difficult to sufficiently reduce the “charging non-uniformity”.
That is, in a constitution in which a combined surface potential is formed by superimposing a charge potential formed by a second charger on a charge potential formed by a first charger, a relationship between the charge potentials formed by the respective chargers is important to make a finally formed charge potential of the photosensitive member uniform. In the case where a photosensitive member having large electrostatic capacity and large dark decay is used or in the like case, the relationship between the charge potentials by the first and second chargers is deviated from a predetermined range and the charge potential of the photosensitive member cannot be made uniform in some instances. Particularly, when the charge potential formed by the first charger exceeds a value of a voltage applied to the grid electrode of the second charger, it becomes difficult to control the charge potential of the photosensitive member by the second charger, so that the “charging non-uniformity” increases.
When a relationship between the aperture ratios of the upstream side grid and the downstream side grid is only defined as described in JP-A 2005-84688, it is insufficient as a counter measure against the above-described problem. Further, the constitution of Japanese Patent No. 5382409 is a constitution such that a single common grid electrode is provided for the two discharging wires, and therefore, a relationship between the charge potential formed on the upstream side and the charge potential formed on the downstream side cannot be properly controlled, so that the reduction in “potential non-uniformity” becomes insufficient.